Lithographic, flexographic, and gravure printing techniques have been refined and improved for many years. The basic principle of lithography is transferring ink from a surface having both ink-receptive and ink-repellent areas that comprise an image. Offset printing incorporates an intermediate transfer of the ink. In offset printing, an offset lithographic press transfers ink from a plate on a rotating cylinder to a rubber blanket cylinder, and then the blanket cylinder transfers the image to a substrate, which may be either a cut sheet or a web substrate. In flexographic printing, the ink is picked up in ink pockets on an anilox roll and transferred to a rubber plate having raised image areas that is mounted on a rotating cylinder. The flexographic plate then transfers the image to a sheet or web substrate. In gravure printing, engraved ink wells are arranged on a cylinder to form an image. When the ink wells contain ink and make direct contact with a sheet or web substrate, an ink image is transferred from the cylinder onto the substrate. The flexographic and gravure methods are especially useful for printing onto a web of film or foil material. After printing, the web material may be cut into sections that are formed into containers, such as bags, for food products, such as potato chips. For high durability images, the printing may be done with UV curable inks using UV-flexo techniques. Following transfer of the UV ink image from a plate on a rotating cylinder to a substrate, the ink image is cured by exposing the image to UV light. Typically, each color image is cured before the next color image is applied to the substrate.
The methods of printing described above are limited by the requirement that a cylinder or other ink transfer member be produced with ink receptive and ink repellant areas, or with raised or depressed areas, or with ink receptive pockets, for the collection of ink to transfer the ink to a substrate and form an image. Thus, these methods are particularly adapted for printing an image numerous times. If the printed image is to have a short run, such as a single copy of the image, then digital printing techniques are more advantageous. Ejecting inks from a print head is one method of digital printing that is well developed.
Ejecting UV curable inks, which have a sufficiently low viscosity that enables the ink to be jetted from a print head, onto porous substrates, such as plain paper, will generally, at room temperatures, result in rapid lateral and depth penetration of the ink into the substrate. These results produce poor edge acuity and showthrough of the images. Therefore, UV gel inks have been developed. These gel inks are relatively solid. That is, these inks have a viscosity between 105 and 107 cps at temperatures below a threshold, such as 75° C. When heated to temperatures above the threshold, these gel inks become liquid and are capable of being ejected from a print head. The inks then freeze into a gel state on contact with the substrate, which prevents spreading of the ink along the substrate and penetration of the ink into the substrate. Consequently, different images can be printed by controlling the ejectors in the print head in a manner known for ink jet and solid ink printers. The ejected gel ink is then cured on the image substrate by exposure to ultraviolet (UV) light.
UV gel inks are described in Copending Application U.S. Ser. No. 11/290,202, filed Nov. 30, 2005, entitled “Phase Change Inks Containing Photoinitiator With Phase Change Properties and Gellant Affinity,” with the named inventors Peter G. Odell, Eniko Toma, and Jennifer L. Belelie, the disclosure of which is completely incorporated herein by reference. That application discloses a phase change ink comprising a colorant, an initiator, and an ink vehicle. The ink vehicle comprises (a) at least one radically curable monomer compound, and (b) a compound of the formula:
wherein R1 is an alkylene, arylene, arylalkylene, or alkylarylene group, R2 and R2′ each, independently of the other, are alkylene, arylene, arylalkylene, or alkylarylene groups, R3 and R3′ each, independently of the other, are either (a) photoinitiating groups, or (b) groups which are alkyl, aryl, arylalkyl, or alkylaryl groups, provided that at least one of R3 and R3′ is a photoinitiating group, and X and X′ each, independently of the other, is an oxygen atom or a group of the formula —NR4—, wherein R4 is a hydrogen atom, an alkyl group, an aryl group, an arylalkyl group, or an alkylaryl group.
Copending Application U.S. Ser. No. 11/290,121, filed Nov. 30, 2005, entitled “Phase Change Inks Containing Curable Amide Gellant Compounds,” with the named inventors Eniko Toma, Jennifer L. Belelie, and Peter G. Odell, the disclosure of which is completely incorporated herein by reference, describes a phase change ink comprising a colorant, an initiator, and a phase change ink carrier. The phase change ink carrier of that application comprises at least one radically curable monomer compound and a compound of the formula:
wherein R1 and R1′ each, independently of the other, is an alkyl group having at least one ethylenic unsaturation, an arylalkyl group having at least one ethylenic unsaturation, or an alkylaryl group having at least one ethylenic unsaturation, R2, R2′, and R3 each, independently of the others, are alkylene groups, arylene groups, arylalkylene groups, or alkylarylene groups, and n is an integer representing the number of repeat amide units and is at least 1.
Copending Application U.S. Ser. No. 11/289,615, filed Nov. 30, 2005, entitled “Radiation Curable Ink Containing A Curable Wax,” with the named inventors Jennifer L. Belelie, et al., the disclosure of which is completely incorporated herein by reference, describes a radiation curable ink comprising a curable monomer that is liquid at 25° C., curable wax and colorant that together form a radiation curable ink. This ink may be used to form images by providing the radiation curable ink at a first temperature; applying the radiation curable ink to the substrate to form an image, the substrate being at a second temperature, which is below the first temperature; and exposing the radiation curable ink to radiation to cure the ink.
In summary, the UV gel inks described above may be used to form images on paper webs or sheets as well as on film or foil webs or sheets. Showthrough or bleeding occurs when a liquid ink penetrates a porous image substrate, such as paper. Gel inks do not penetrate porous substrates as they cool and return to gel form following ejection from the heated print head. Thus, showthrough or bleeding is prevented. Additionally, gel ink ejected onto a porous substrate can be more thoroughly cured by UV light because the gel ink does not penetrate the porous substrate so fibers of the porous substrate cannot shade the ink from the light. Uncured or incompletely cured ink is undesirable because it is still susceptible to smudge and still capable of releasing odors.
While gel ink enables more facile image printing and printing onto porous substrates, it has been observed as exhibiting microbanding. Microbanding is an uneven distribution of ink in an image area in which the image should be smooth and uniform. Because the ink temperature drops after ejection, the ink freezes on contact with the substrate and an uneven distribution of ink on the image substrate may occur. The uneven distribution can sometimes be observed by the human eye as bands or lines in the direction of the substrate travel past the print head. This uneven distribution might be addressed by leveling the ink on the image substrate with a contact member, such as a roller, belt, or wiper, in an effort to normalize the ink distribution. A heating element may be located near or within the contact member to heat it and consequently soften the ink for the leveling operation.
Leveling the gel ink with a contact member may cause the ink layer to split, however. A portion of the gel ink may be transferred to the contact member and affect the print quality of later processed images. For example, a portion of the ink transferred from a rotating contact member may later be deposited on the media to leave a ghost of the previously leveled image. Further, ink build up on a contact member necessitates either replacement of the contact member or removal of the ink from the contact member on a periodic or occasional basis. Consequently, addressing the microbanding defect of gel ink in an image without splitting the ink or accumulating ink on a contact member would be useful.